Earlier quoted context omitted.
A vaccine doesn't work unless it helps the tiny fraction of fentenyl deaths who unintentnially take the drug? How about this: You're less likely to come in contact with the drug if there are fewer users getting high off it.
Is there some group of people who are taking fentanyl en masse intentionally? I know a few people who have died after taking what they thought were bars or ecstasy and died because it was laced with fentanyl. I know 0 who go out of their way to find and take fentanyl.
It makes at least some sense given the common narrative of the pipeline from painkiller addiction to heroin (i.e. it's the same relationship - people who are addicted to painkillers may transition to heroin because it's cheaper and more powerful).